This invention relates to a thermal head and particularly to a thermal head capable of printing multigradational tones.
FIG. 1 shows the structure of a one-dot element of a conventional black-and-white binary thermal head wherein FIG. 1a is a plan view thereof, FIG. 1b is a cross-sectional diagram taken along line A--A in FIG. 1a, and FIG. 1c is a graph of the applied energy vs. printed dot area characteristic of the heating resistor. As illustrated in FIG. 1b, on an insulating substrate 3 made of ceramic, glass or the like, are formed in turn a resistive layer 2 which is made of a semiconductor alloy such as CrSi (chromium-silicon) and has a substantially constant thickness, and a pair of opposite electrodes 1,1' made of a conductive material such as aluminum or chromium. The resistor of the resistive layer 2 lying between the electrodes 1,1' generates heat when supplied with electric power through the electrodes 1,1', and thus it is called a heating resistor 2.
For printing with a thermal head, there are widely used the thermal type which employs thermal paper and the thermal transfer type in which the thermal head is pressed against a film of which the rear surface is coated with ink and thereby transfers an image to a sheet of ordinary paper disposed under the film. The heating resistor 2 with a constant width and thickness as shown in FIG. 1 generates heat uniformly over its surface. FIG. 1c shows the printing characteristic of the heating resistor 2 of such structure. The abscissa indicates the energy relative to the energy necessary for printing one dot of substantially the same area as the surface area of the heating resistor 2 which energy is taken as unity for comparison, and the ordinate indicates the dot area relative to the surface area of the heating resistor 2 which surface area is taken as unity for comparison. From curve 25 in FIG. 1c it will be seen that the heating resistor does not start to print until the applied energy P increases and exceeds a constant amount of energy Est. This energy Est is called the printing start energy. The printing start energy Est is dependent on physical constants such as the shape, size, thermal capacity, thermal conductivity and melting points of the heating resistor, substrate and protective film made of ceramic and glass, thermal paper and ink film, ambient temperature, and so on. Particularly this printing start energy Est is greatly dependent on the size of the heating resistor and the recording type. Therefore, it is possible to estimate the printing start energy Est from the selected recording type, and the physical constants of the thermal paper or ink film.
The printing around the printing start energy Est is very unstable because the printed dot area S is changed by the condition in which the thermal head contacts the recording paper, and by the irregularity of the surface of the recording paper, lack of uniformity in the ingredients mixed in the ink and so on. Therefore, an unstable region occurs as shown by the hatched area.
In the heating resistor 2 of the uniformly heat-generating structure shown in FIGS. 1a and 1b, the unstable condition occurs over the whole resistor and thus it is not possible to stably print dots of an intermediate-level area. For this reason, this heating resistor is not suitable for the conventional halftone printing method of printing smaller dot areas than the surface area of the heating resistor 2.
Moreover, in this heating resistor 2, when the applied energy is increased to exceed the energy E 1.0 at which the average dot area substantially equals the surface area of the heating resistor 2 (i.e., S=1), almost no unstable region occurs, or the printing condition enters into the stable printing region, in which stable printing is possible. In the uniformly heating resistor 2 shown in FIGS. 1a and 1b, however, the dot area is not so greatly changed in the stable printing region (S.gtoreq.1) and thus no multigradation can be achieved.
FIG. 2 shows the structure of the heating resistor 4 of another conventional thermal head capable of halftone wherein FIG. 2a is a plan view thereof, and FIG. 2b is a cross-sectional diagram taken along line B--B in FIG. 2a. The thermal head of this type was disclosed in Japanese Patent Laid-open No. 161947/1979.
The structure of a plurality of thermal head having the heating resistors 4 shown in FIG. 2 is substantially the same as that of a plurality of conventional binary head having the heating resistors 2 shown in FIG. 1, but the shape of its heating resistor 4 is different from that of the heating resistor 2. The resistor 4 has a constant thickness as shown in FIG. 2b, but its width continuously varies to be smallest at the center and to be the larger at places nearer to either of the electrodes as shown in FIG. 2a. The heating resistor 4 of this structure has a higher current density at the center and thus generates more heat at the center than at the periphery. Therefore, when little energy is applied to the resistor 4, only the center portion of the resistor 4 prints a smaller dot. Moreover, as the applied energy increases, the peripheral portion of the resistor 4 becomes able to print and hence the printed dot area increases. Thus, a halftone picture can be reproduced by controlling the amount of energy to be applied to this head on the basis of gradational data of the picture.
Since the sensitivity of a human's eye to a halftone picture generally becomes high in a low-optical-density range, it is most important to consider the halftone printing ability of the thermal head in the low-optical-density range. In other words, if thermal head meets the requirements that the minimum printed dot area is small, and that the printed dot area is stable with respect to the applied energy, the thermal head can be said to be suitable for printing the halftone. However, it is difficult to control the heating resistor 4 of the thermal head as shown in FIG. 2a for halftone printing for the following reasons.
FIG. 3 shows the printing characteristic of the conventional heating resistor shown in FIG. 2 for halftone printing. FIG. 3a is a plan view of a half of the heating resistor 4. The half of the resistor 4 as illustrated is equally partitioned along line B--B in FIG. 2a, into 100 parts for the purpose of showing the characteristics of the thermal head. FIG. 3b is a graph of the printing characteristic of each of the divided parts of the resistor 4, and FIG. 3c is a graph of measured dot areas and standard deviation values showing the stability of the dot area with respect to the applied energy.
If the 100 divided parts of the half of resistor 4 are represented by R.sub.1, R.sub.2, R.sub.3 . . . R.sub.99 and R.sub.100 in the order of width as shown in FIG. 3a, the printing characteristics of R.sub.1, R.sub.2 . . . R.sub.99 and R.sub.100 are respectively given by S.sub.1, S.sub.2, S.sub.3 . . . S.sub.99 and S.sub.100 as shown in FIG. 3b. Each of many divided parts of the resistor has an unstable region as shown by the hatched area in FIG. 3b because it almost uniformly generates heat. In addition, as shown by the characteristics S.sub.1, S.sub.2, S.sub.3 the unstable regions of the adjacent printing characteristics are overlapped, and thus the unstable region always exists until the applied energy P exceeds 1.0 where all the resistors R.sub.1, R.sub.2 . . . R.sub.100 reach their stable regions. Moreover, the printed dot area greatly scatters around the average dot area S when most resistor parts are in their unstable regions at low applied energy, or when the gradation printing is made at a low-optical-density.
FIG. 3c shows the dot area and standard deviation for the stability of dot area with respect to the applied energy. The characteristic curves in FIG. 3c were determined by the experiment on the conventional halftone thermal head element shown in FIG. 2. The abscissa indicates the energy relative to the energy necessary for printing substantially the same area as the surface area of the heating resistor 4 which is "1", the left ordinate shows the printed dot area relative to the surface area of the heating resistor 4, and the right ordinate indicates the standard deviation normalized by dividing by the dot area S (hereinafter, simply called the standard deviation). The greater the standard deviation, the more unstable the printing characteristic, and hence the lower the halftone printing ability. An experiment revealed that the halftone printing ability was greatly reduced when the standard deviation of the dot area exceeds 1. The solid curve, 11 in FIG. 3c indicates the dot area with respect to the applied energy and the broken line, 12 therein shows the standard deviation of the dot area. From FIG. 3c, it will be seen that in the conventional halftone thermal head, the printing resistor 4 has a standard deviation higher than 1 and hence low halftone ability when it prints a dot area smaller than the surface area of the heating resistor 4. The reason for this will be described with reference to FIG. 4.
FIG. 4 shows the state in which proper electric energy is applied to the heating resistor 4 of the conventional halftone thermal head. The center portion, 13 of the heating resistor 4 is supplied with great energy per unit area and thus can print positively. The portions 14,14' adjacent to the center 13 are supplied with insufficient energy and hence print unstably. The paired portions 15,15' adjacent to the electrodes 1,1' are supplied with little energy and hence cannot print.
Thus, when the heating resistor 4 of this halftone thermal head prints a dot of an area smaller than the surface area of the heating resistor 4, the unstable printing regions of the portions 14,14' are always involved in the printing, and hence make the printing characteristics unstable. Particularly, the unstable printing regions degrade the printing quality of the low-optical-density gradation which needs to stably print very small dots.